Epidemic dengue caused by one or more of the four dengue virus serotypes continues to pose a major public health problem in most tropical and subtropical areas. A safe and effective dengue vaccine against dengue is currently still not available. Earlier, DEN4 mutant 3?d (172-143) containing a deletion in the 3' non-coding (NC) region that exhibited reduced replicative capacity in simian cell culture and in primates was selected for further evaluation in humans. Following a single dose inoculation, all 20 volunteers in three cohorts remained afebrile and exhibited very few symptoms of dengue clinical signs, except for a transient rash. Three vaccinees showed an elevation of liver enzyme ALT. Importantly, each of the vaccinees analyzed thus far developed a high titer of DEN4 neutralizing antibodies at four to six weeks after immunization. However, a small number of vaccinees showed an elevation of liver enzyme ALT. This indicates that there remains a concern about the safety for the use of a live dengue virus vaccine. Passive immunization with dengue virus neutralizing antibodies provides an attractive alternative to prevention of dengue virus infection. Toward this goal, we employed the technique of phage display of combinatorial antibody libraries that has provided a power tool for isolation of human or chimpanzee antibodies to important viral pathogens. Panning of the phage library and subsequent screening has allowed identification of a number of E. coli colonies producing Fab fragments that were reactive to DEN4 virions. The purified Fab fragment from each of these clones failed to exhibit demonstrable DEN4 neutralizing activity. Each of the recombinant Fab?s was then converted into whole IgG expressed in mammalian cells. Importantly, each of these monoclonal antibodies exhibited DEN4 neutralizing activity by plaque reduction neutralization assay. These are the first monoclonal DEN4 neutralizing antibodies cloned from primates (chimpanzees) that are most closely related to humans. Because the close homology between chimpanzees and humans, it is likely that the chimpanzee antibodies would not be immunogenic in humans and might be useful in immunoprophylaxis against dengue.